Positive electrode active material for lithium ion secondary batteries and method for producing same

ABSTRACT

The production method is a method for producing a positive electrode active material for a lithium ion secondary battery which contains at least nickel and lithium, the method including: a firing process in which a mixture of a nickel compound powder and a lithium compound powder is fired; and a water washing process in which a lithium-nickel composite oxide powder obtained in the firing process is washed with water, wherein the firing process is performed under conditions such that a value obtained by dividing a ratio of an amount-of-substance of lithium to a total amount-of-substance of transition metals other than lithium in the lithium-nickel composite oxide powder after the washing with water by a ratio of an amount-of-substance of lithium to a total amount-of-substance of transition metals other than lithium in the lithium-nickel composite oxide powder before the washing with water exceeds 0.95.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a lithium ion secondary battery and a method for producing the same,and particularly relates to a positive electrode active material for ahigh-output lithium ion secondary battery which includes alithium-nickel composite oxide and a method for producing the same.

BACKGROUND ART

Along with a recent rapid expansion of production of compact informationterminals such as smartphones and tablet terminals, demands for lithiumion secondary batteries as high-capacity secondary batteries haverapidly increased. Since lithium ion secondary batteries are compact andhave a high energy density, they have already been widely used as powersources for compact information terminals, and research and developmentof their application as large power sources to be installed in hybridcars and electric cars are under progress.

As positive electrode active materials for use in lithium ion secondarybatteries, lithium-transition metal composite oxides are widely used.Particularly, a lithium-cobalt composite oxide (LiCoO₂) that canrelatively easily be synthesized has heretofore been mainly used.However, since a rare and expensive cobalt compound is used as a rawmaterial to synthesize the lithium-cobalt composite oxide, the cost ofthe raw material of positive electrode active materials increases, whicheventually increases prices of lithium ion secondary batteries. Areduction in the raw material costs of positive electrode activematerials in order to achieve the production of less-expensive lithiumion secondary batteries is thus highly significant from an industrialviewpoint because this leads to a reduction in the costs of secondarybatteries now commonly used for compact information terminals or cars,and further makes it possible to install lithium ion secondary batteriesin future large power sources.

Under the circumstances, a lithium-nickel composite oxide (LiNiO₂) hasattracted attention as another lithium-transition metal composite oxidethat can be used as a positive electrode active material for a lithiumion secondary battery. A lithium-nickel composite oxide has advantagesthat it has a larger charge and discharge capacity per mass than alithium-cobalt composite oxide that is now dominantly used and that itis made from a nickel compound which is less expensive and stablyavailable as compared with a cobalt compound. Therefore, alithium-nickel composite oxide has been expected as a next-generationpositive electrode active material and thus it is under active researchand development.

However, a lithium-nickel composite oxide has the following problem: alithium-nickel composite oxide is poor in productivity in industrialmass production because it is decomposed by elimination of oxygen from acrystal at a lower temperature than a lithium-cobalt composite oxide,and therefore the temperature of a synthesis reaction between a lithiumcompound and a nickel compound cannot be increased, and firing timeneeds to be increased to allow the reaction to sufficiently proceed toform organized crystal.

Patent Literatures 1 to 4 disclose methods for producing thelithium-nickel composite oxide, in which a lithium compound and a nickelcompound are mixed and subjected to thermal treatment in order toimprove battery characteristics. These Literatures 1 to 4 proposeoptimization of the time and temperature of a synthesis reaction and thecomposition of an atmosphere gas for the synthesis reaction. Further,Patent Literatures 5 to 7 propose various synthesis methods includingfiring of a lithium-transition metal composite oxide. Further, PatentLiterature 8 discloses a method including a water washing process toimprove the properties of a positive electrode active material.

CITATION LIST Patent Literatures

Patent Literature 1: JP 7-114915 A

Patent Literature 2: JP 11-111290 A

Patent Literature 3: JP 2000-133249 A

Patent Literature 4: JP 2007-119266 A

Patent Literature 5: JP 2002-170562 A

Patent Literature 6: JP 2000-173599 A

Patent Literature 7: JP 2008-117729 A

Patent Literature 8: JP 2011-146309 A

SUMMARY OF INVENTION Technical Problem

However, it is difficult for the techniques disclosed in PatentLiteratures 1 to 8 to industrially and stably mass-produce a positiveelectrode active material for a lithium ion secondary battery which isexcellent in battery performance. Under the above circumstances, it isan object of the present invention to provide a lithium-nickel compositeoxide excellent in battery performance as a positive electrode activematerial for a high-output lithium ion secondary battery by a methodcapable of industrially and stably mass-producing it.

Solution to Problem

In order to achieve the above object, the present inventors have studiedthe synthesis of a positive electrode active material, and as a resulthave found that a positive electrode active material having stablebattery performance can industrially be mass-produced by controlling theratio of the amount-of-substance of lithium contained in alithium-nickel composite oxide before washing with water to that afterwashing with water in a positive electrode active material productionmethod including: a firing process in which a powder raw materialcontaining a nickel compound and a lithium compound is fired to generatea lithium-nickel composite oxide; and a water washing process in whichthe composite oxide is washed with water after the firing process. Thisfinding has led to the completion of the present invention.

More specifically, the present invention is directed to a method forproducing a positive electrode active material for a lithium ionsecondary battery which contains at least nickel and lithium, the methodincluding: a firing process in which a mixture of a nickel compoundpowder and a lithium compound powder is fired; and a water washingprocess in which a lithium-nickel composite oxide powder obtained in thefiring process is washed with water, wherein the firing process isperformed under conditions such that a value obtained by dividing aratio of an amount-of-substance of lithium to a totalamount-of-substance of transition metals other than lithium in thelithium-nickel composite oxide powder after the washing with water by aratio of an amount-of-substance of lithium to a totalamount-of-substance of transition metals other than lithium in thelithium-nickel composite oxide powder before the washing with waterexceeds 0.95.

The present invention is also directed to a positive electrode activematerial for a lithium ion secondary battery, the positive electrodeactive material including a lithium-nickel composite oxide, wherein whenthe positive electrode active material is washed with water by adding0.75 mass parts of the positive electrode active material to 1 mass partof pure water to prepare a mixture, stirring the mixture for 30 minutes,and drying solid matter obtained by subjecting the mixture tosolid-liquid separation, a value obtained by dividing a ratio of anamount-of-substance of lithium to a total amount-of-substance oftransition metals other than lithium in the lithium-nickel compositeoxide powder after the washing with water by a ratio of anamount-of-substance of lithium to a total amount-of-substance oftransition metals other than lithium in the lithium-nickel compositeoxide powder before the washing with water exceeds 0.95.

Advantageous Effects of Invention

According to the present invention, it is possible to industriallymass-produce a positive electrode active material having stable batteryperformance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic explanatory diagram showing a measurement examplefor impedance evaluation and an equivalent circuit used for analysis.

FIG. 2 shows a schematic perspective view and a cross sectional view ofa coin-type battery produced for battery evaluation in Examples of thepresent invention.

FIG. 3 is a graph obtained by plotting the ratios between lithium metalratios before and after washing with water and the initial dischargecapacities of various lithium-nickel composite oxides produced inExamples of the present invention.

FIG. 4 is a graph obtained by plotting the ratios between lithium metalratios before and after washing with water and the positive electroderesistances of various lithium-nickel composite oxides produced inExamples of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, an embodiment of a method for producing a positiveelectrode active material for a lithium ion secondary battery accordingto the present invention will be described in detail. The method forproducing a positive electrode active material according to theembodiment of the present invention includes: a firing process in whicha mixture of a nickel compound powder and a lithium compound powder isfired; and a water washing process in which a lithium-nickel compositeoxide powder obtained in the firing process is washed with water. When alithium-nickel composite oxide powder used as a positive electrodeactive material for a lithium ion secondary battery is industriallyproduced, generally, a mixture of a nickel compound powder and a lithiumcompound powder is filled in ceramic firing containers, and then thecontainers filled with the mixture are continuously introduced into acontinuous firing furnace such as a roller hearth kiln or a pusherfurnace to perform firing at a predetermined temperature for apredetermined time. This causes a synthesis reaction so that alithium-nickel composite oxide powder is generated from the mixture.

A ceramic rectangular container is used for the firing container inapplication of industrial production, which generally has insidedimensions ranging from 100 mm(L)×100 mm(W)×20 mm (H) to 500 mm(L)×500mm(W)×120 mm(H). This container is filled with a mixture of a lithiumcompound powder and a nickel compound powder as raw materials so thatthe height of the mixture from the bottom surface of the container fallsin the range of 10 to 110 mm.

In order to improve the productivity of the firing process, the amountof the mixture fired per unit time may be increased by increasing theconveying speed of the firing container in the continuous firing furnaceto reduce firing time or by increasing the amount of the mixture filledin the firing container. However, if the conveying speed is too high,the following problem may arise: the time for a synthesis reactionbetween the nickel compound powder and the lithium compound powder isshort, and therefore battery performance is degraded due to insufficientcrystal growth of particles constituting the lithium-nickel compositeoxide powder. On the other hand, if the amount of the mixture filled inthe firing container is too large, the depth from the top surface of themixture filled in the container to the bottom of the container is toodeep, and therefore oxygen required for the reaction is not sufficientlydistributed to the mixture present near the bottom of the container. Asa result, the synthesis reaction represented by the following formula 1is less likely to successfully proceed, and therefore a problem such asa reduction in discharge capacity may occur due to insufficientsynthesis of a lithium-nickel composite oxide.NiO+LiOH+¼O₂→LiNiO₂+½H₂O  [Formula 1]

Therefore, in order to synthesize a lithium-nickel composite oxidepowder that achieves excellent battery performance as a positiveelectrode active material, it is preferred that oxygen required for thereaction between the nickel compound powder and the lithium compoundpowder is sufficiently distributed throughout the mixture of them, andis particularly distributed to the bottom of the firing container. Inthis regard, the present inventors have found that in the reactionbetween the nickel compound powder and the lithium compound powder,oxygen does not need to sufficiently be distributed throughout themixture over the full temperature range in the firing process, andoxygen can sufficiently be distributed throughout the mixture as long asoxygen diffusion time depending on the depth of the mixture in thefiring container can be secured in a temperature range important for thereaction, specifically in the firing temperature range of 450° C. orhigher and 650° C. or lower during firing of the mixture, in which casea lithium-nickel composite oxide powder having excellent batteryperformance can be obtained even when the amount of the mixture filledin the firing container is large, and therefore the depth from the topsurface of the mixture to the bottom of the container is deep.

More specifically, depending on the kind of lithium compound, a solidphase-solid phase reaction or a liquid phase-solid phase reactionbetween a lithium compound and a nickel compound most significantlyproceeds in the firing temperature range of 450° C. or higher and 650°C. or lower. Therefore, when oxygen required for the reaction issufficiently distributed throughout the mixture in such a temperaturerange, a sufficient reaction between the lithium compound and the nickelcompound can be provided to obtain a lithium-nickel composite oxidepowder. It is to be noted that the firing temperature can be measuredby, for example, a thermometer provided in the furnace.

For example, when a mixture of a lithium hydroxide powder and a nickelcomposite oxide powder is prepared as a raw material, and thetemperature of the mixture is gradually increased, a reaction for thesynthesis of a lithium-nickel composite oxide starts at about 450° C.When the temperature exceeds about 460° C. that is the melting point oflithium hydroxide, lithium hydroxide melts and reacts with a nickelcomposite oxide. If oxygen is not sufficiently distributed to the bottomof the firing container in this temperature range, unreacted meltedlithium hydroxide reacts with the ceramic firing container so that theamount of lithium hydroxide to be reacted with a nickel composite oxidebecomes practically insufficient. As a result, lithium-deficient sitesare present in a generated lithium-nickel composite oxide, which causesdegradation of battery performance. Therefore, it is important tosufficiently supply oxygen to the mixture to allow the reaction toremarkably proceed when the firing temperature during firing is 450° C.or higher that is a temperature range in which a sufficient reactionrate is achieved due to melting of lithium hydroxide.

On the other hand, if unreacted lithium hydroxide and nickel compositeoxide are still present and supply of oxygen to them is insufficienteven after the firing temperature reaches 650° C., a side reactionrepresented by the following formula 2 occurs so that a different phasethat prevents the migration of lithium ions during a battery reaction isgenerated in a generated lithium-nickel composite oxide crystal, whichcauses degradation of battery performance.8NiO+2LiOH+½O₂→Li₂Ni₈O₁₀+H₂O  [Formula 2]

It is to be noted that as described above, when the firing temperatureis in the range of 450° C. or higher and 650° C. or lower, there islittle difference in the effect between when the firing temperature ismaintained at a predetermined value and when the firing temperature ischanged at, for example, a certain temperature rise rate, and the sameeffect can be obtained in both cases. Further, the reaction representedby the above formula 1 basically requires oxygen, and therefore anatmosphere gas during firing may be air having an oxygen concentrationof about 20 vol %, but is preferably oxygen-rich air having a higheroxygen concentration. The oxygen concentration of the oxygen-rich air ispreferably set to 60 vol % or more, more preferably 80 vol % or more by,for example, appropriately adjusting the mixing ratio between air andoxygen.

In order to ensure sufficient crystallinity in the synthesis of alithium-nickel composite oxide to achieve high battery performance, themaximum firing temperature in the firing process is preferably 650° C.or higher and 850° C. or lower, the maximum firing temperature ispreferably maintained for 2 hours or longer. If the maximum firingtemperature is lower than 650° C., or the holding time to maintain themaximum firing temperature is shorter than 2 hours even when the maximumfiring temperature is 650° C. or higher, the thereby-obtainedlithium-nickel composite oxide may have insufficient crystallinity. Onthe other hand, if the maximum firing temperature exceeds 850° C., thethereby-obtained lithium-nickel composite oxide starts its decompositionreaction associated with oxygen release which may degrade batteryperformance due to disruption of a layered structure.

Even when the maximum firing temperature is lower than 650° C.,prolonged firing can synthesize a lithium-nickel composite oxide havingsufficient crystallinity without impairing battery performance. However,the firing time exceeding 24 hours is not preferable in consideration ofindustrial productivity. Therefore, the time required for the firingcontainer filled with the mixture to pass through the firing furnace,that is, the time from the start of heating through temperature rise tothe maximum firing temperature and maintaining at the maximum firingtemperature to the completion of cooling is preferably 24 hours orshorter.

It is to be noted that when lithium hydroxide having water ofcrystallization is used as the lithium compound, a rapid increase of thefiring temperature causes non-uniform temperature distribution of themixture in the firing container which may lead to uneven synthesisreaction. Therefore, the firing time from the start of heating to thecompletion of maintaining at the maximum firing temperature ispreferably 12 hours or longer. On the other hand, the use of anhydrouslithium hydroxide can shorten the time from the start of heating to thecompletion of maintaining at the maximum firing temperature to less than12 hours.

The present inventors have found that when the lithium-nickel compositeoxide powder is washed with water under predetermined conditions, avalue obtained by dividing the ratio of the amount-of-substance oflithium to the total amount-of-substance of transition metals other thanlithium in the lithium-nickel composite oxide powder after the washingwith water by the ratio of the amount-of-substance of lithium to thetotal amount-of-substance of transition metals other than lithium in thelithium-nickel composite oxide powder before the washing with water(hereinafter, also referred to as a ratio between lithium metal ratiosbefore and after washing with water) can effectively be used as an indexfor determining whether or not the above-described reaction for thesynthesis of a lithium-nickel composite oxide from a nickel compound anda lithium compound has sufficiently proceeded. It is to be noted thatthe amount-of-substance is a physical amount expressed by mole that isthe SI unit, and can be measured by an ICP emission spectrophotometer orthe like.

In the production of a lithium-nickel composite oxide, a nickel compoundand a lithium compound are usually mixed so that the amount-of-substanceof lithium in the lithium compound is larger than 1 mole per mole of theamount-of-substance of nickel and an additive transition metal elementcontained in the nickel compound, and the mixture is fired. This isbecause if the amount-of-substance of lithium is smaller than theamount-of-substance of nickel and an additive transition metal elementin a reaction for the synthesis of a lithium-nickel composite oxide,lithium deficiency may occur at lithium sites in the crystal of thelithium-nickel composite oxide, which results in an insufficient chargeand discharge capacity, and the lithium sites with the lithiumdeficiency function as resistive layers in a charge and dischargereaction which increases the resistance of a secondary battery.

Therefore, even when a sufficient reaction for the synthesis of alithium-nickel composite oxide occurs, the surplus lithium compound isstill present in the lithium-nickel composite oxide after the synthesisreaction, and most of the surplus lithium compound is present on thesurfaces and their vicinity of lithium-nickel composite oxide particles.The surplus lithium compound present on the surfaces and their vicinityof the particles is easily removed by washing with water, and thereforethe ratio between lithium metal ratios before and after washing withwater is usually less than 1.

On the other hand, when the ratio between lithium metal ratios beforeand after washing with water is close to 1, that is, when there islittle difference between lithium metal ratios before and after washingwith water, it can be considered that the amount of surplus lithiumpresent on the surfaces of particles of the lithium-nickel compositeoxide after the synthesis reaction is small. This situation can beresult from a sufficient reaction of lithium used as a raw materialwhich has been mostly solid-solved in nickel compound particles, suchthat an almost stoichiometric reaction for the synthesis of alithium-nickel composite oxide has proceeded.

Therefore, in the method for producing a positive electrode activematerial including a lithium-nickel composite oxide according to theembodiment of the present invention, the ratio between lithium metalratios before and after washing with water is adjusted to exceed 0.95.The ratio between lithium metal ratios is preferably 0.99 or more, morepreferably 0.995 or more to achieve more excellent batterycharacteristics. It is to be noted that even though the upper limit ofthe ratio between lithium metal ratios before and after washing withwater is usually less than 1 as described above, an insufficientreaction for the synthesis of a lithium-nickel composite oxide may causeelution of nickel and an additive transition metal element, which mayresult in a case where the ratio between lithium metal ratios before andafter washing with water is larger than 1. Therefore, the ratio betweenlithium metal ratios before and after washing with water does not exceed1 as long as the lithium-nickel composite oxide is generated by asufficient synthesis reaction.

When the ratio between lithium metal ratios before and after washingwith water is 0.95 or less, it can be made larger than 0.95 byappropriately adjusting conditions for firing in the previous process.For example, a nickel compound powder having a smaller volume-averageparticle diameter MV may be used as a raw material, or a nickel compoundpowder roasted at a higher temperature may be used as a raw material.Alternatively, the oxygen concentration of an atmosphere gas duringfiring may be increased, the maximum firing temperature during firingmay be increased, or the holding time to maintain the maximum firingtemperature may be increased.

In the embodiment of the method for producing a positive electrodeactive material according to the present invention, the nickel compoundpowder used as a raw material is not particularly limited, but nickelhydroxide or nickel oxide is preferred from the viewpoint that sidereaction products other than water are less likely to be generatedduring the reaction. The volume-average particle diameter MV of thenickel compound powder is preferably 3 μm or more and 26 μm or less,more preferably 8 μm or more and 21 μm or less, most preferably 10 μm ormore and 16 μm or less. On the other hand, the bulk density of thenickel compound powder is preferably 0.5 g/ml or more and 2.2 g/ml ofless.

If the volume-average particle diameter MV of the nickel compound powderis less than 3 μm, a sufficient packing density cannot be achieved whenan electrode is produced due to too small a particle diameter of aresulting positive electrode active material, which may result in areduced battery capacity of a secondary battery due to a smaller amountof the positive electrode active material per unit volume of thesecondary battery. On the other hand, if the volume-average particlediameter MV of the nickel compound powder exceeds 26 μm, the number ofcontact points between positive electrode active material particles orbetween positive electrode active material particles and a conductiveaid is too small, which may result in a reduced battery capacity due toan increase in the resistance of a positive electrode.

If the bulk density of the nickel compound powder is less than 0.5 g/ml,the amount of the mixture of the lithium compound powder and the nickelcompound powder filled in the firing container for firing processbecomes smaller due to too low a bulk density of the mixture, which maysignificantly reduce the productivity. On the other hand, if the bulkdensity exceeds 2.2 g/ml, the mixture of the lithium compound powder andthe nickel compound powder is densely filled, which may result in asituation where oxygen is less likely to diffuse into the mixture sothat productivity is reduced due to an increase in time required forfiring.

The carbon content of the nickel compound powder is preferably 0.25 mass% or less, more preferably 0.15 mass % or less, most preferably 0.07mass % or less. As described above, when the carbon content of thenickel compound is as small as 0.25 mass % or less, lithium canefficiently be solid-solved in the nickel compound, which makes itpossible to significantly improve the output characteristic of thepositive electrode active material. The reason why the solid-solving oflithium is efficiently achieved is not clear, but the present inventorshave estimated that carbon contained in the nickel compound powder maymainly be derived from nickel carbonate or nickel carbide that is poorerin reactivity with the lithium compound as compared with nickelhydroxide or nickel oxide.

When nickel hydroxide is used as a raw material, the method forproducing a positive electrode active material according to theembodiment of the present invention preferably includes, before thefiring process, a preprocessing process in which the nickel compoundpowder is converted to a nickel oxide powder by roasting at a roastingtemperature of preferably 500° C. or higher and 800° C. or lower. Byroasting the nickel compound powder before firing in this way, it isalso possible to reduce the carbon content of the nickel compoundpowder. It is to be noted that the roasting temperature can be measuredby, for example, a thermometer provided in the furnace.

The lithium compound powder used as a raw material is not particularlylimited, but is preferably lithium hydroxide or lithium carbonate or amixture of them, more preferably anhydrous lithium hydroxide or lithiumhydroxide monohydrate having a melting point of about 480° C. inconsideration of reaction with the nickel compound, and most preferablyanhydrous lithium hydroxide in consideration of productivity. Lithiumhydroxide is preferable because it melts and proceeds a solid-liquidreaction with the nickel composite oxide which allows uniform reaction.

Since the reaction for the synthesis of a lithium-nickel composite oxideis basically a solid-phase reaction, the synthesis reaction proceedsreadily when the lithium compound powder and the nickel compound powderused as raw materials are homogeneously mixed. For this reason, theparticle size of the lithium compound powder is preferably close to thatof the nickel compound powder. As described above, since the particlesize of the nickel compound powder expressed as a volume-averageparticle diameter MV is preferably 3 μm or more and 26 μm or less, thevolume-average particle diameter MV of the lithium compound powder ispreferably 26 μm or less, more preferably 21 μm or less, most preferably16 μm or less. However, if the particle size of the lithium compoundpowder is 5 μm or less, energy required for grinding is too high, whichcauses an increase in cost, and the bulk density of the mixture of theraw materials is too low, which requires a firing furnace having a largecapacity. Therefore, from a practical viewpoint, the volume-averageparticle diameter MV of the lithium compound powder is preferably 5 μmor more, more preferably 10 μm or more.

In the embodiment of the method for producing a positive electrodeactive material according to the present invention, the lithium-nickelcomposite oxide powder obtained by firing is washed with water so that,as described above, surplus lithium present on the surfaces or theirvicinity of the particles is removed, which provides a high-capacity andsafety positive electrode active material for a lithium ion secondarybattery. Conditions for the washing with water are not particularlylimited, and a known water washing technique can be used. However, thewashing with water is preferably performed by the following method.

Specifically, the lithium-nickel composite oxide powder is added in anamount of preferably 0.5 to 2 mass parts, more preferably 0.75 massparts to 1 mass part of water at preferably about 10 to 15° C. containedin a container equipped with a stirrer, and the mixture is stirred forpreferably about 15 to 60 minutes, more preferably about 30 minutes tosufficiently remove surplus lithium present on the surfaces or theirvicinity of the lithium-nickel composite oxide particles. The slurryafter washing with water may be subjected to solid-liquid separation anddrying by a general method. If the amount of the lithium-nickelcomposite oxide powder added exceeds 2 mass parts, the stirring of theslurry may become difficult due an excessive increase of a viscosity ofthe slurry, and further the dissolution rate of adhesive substances maybe reduced by chemical equilibrium due to an increase of alkali contentin the slurry, or separation of the adhesive substances from the powdermay become difficult even when the adhesive substances are peeled off.

On the other hand, if the amount of the lithium-nickel composite oxidepowder added is less than 0.5 mass parts, elution of lithium is promotedbecause the concentration of the slurry is too low, which causeselimination of lithium from the crystal lattice of the lithium-nickelcomposite oxide particles as a positive electrode active material. Thissituation not only easily causes crystal collapse but also causesre-precipitation of lithium carbonate due to absorption of carbondioxide in the atmosphere into a high-pH aqueous solution of the slurry.

Water used in the washing with water is not particularly limited, butpure water having an electric conductivity of less than 10 μS/cm ispreferred, and pure water having an electric conductivity of 1 μS/cm orless is more preferred. More specifically, the use of pure water havingan electric conductivity of less than 10 μS/cm makes it possible toprevent a reduction in battery performance caused by adhesion ofimpurities to the lithium-nickel composite oxide particles as a positiveelectrode active material. After the slurry is subjected to solid-liquidseparation, the amount of adhesion water remaining on the surfaces ofthe particles is preferably small. If the amount of adhesion water islarge, lithium dissolved in the adhesion water is re-precipitated sothat the lithium amount remaining on the surface of the lithium-nickelcomposite oxide powder after subsequent drying increases. Therefore, asolid-liquid separator used for solid-liquid separation performed afterwashing with water is preferably one capable of achieving a low watercontent of solid matter after solid-liquid separation, such as acentrifugal separator or a filter press.

Conditions for drying performed after the solid-liquid separation arenot particularly limited, but a wet powder cake obtained by thesolid-liquid separation is preferably dried at a drying temperature of80° C. or higher and 550° C. or lower using a dryer capable ofcontrolling an atmosphere in its chamber so that the chamber is filledwith a gas atmosphere containing no carbon compound component nor sulfurcompound component or a vacuum atmosphere. The reason why the dryingtemperature during drying is set to 80° C. or higher is to quickly drythe lithium-nickel composite oxide particles after the washing withwater as a positive electrode active material to prevent the generationof concentration gradient of lithium between the surface and inside ofthe particles.

On the other hand, the reason why the drying temperature is set to 550°C. or lower is that it is supposed that the surfaces and their vicinityof the lithium-nickel composite oxide particles as a positive electrodeactive material are in an approximately stoichiometric state or close toa charged state due to slight elimination of lithium, and therefore ifthe drying temperature higher than 550° C. may trigger distortion ofcrystal structure of the powder close to a charged state, which maydeteriorate electrical characteristics. The drying temperature duringdrying is more preferably 120 to 350° C. in consideration ofproductivity and heat energy cost. It is to be noted that the dryingtemperature can be measured by, for example, a thermometer provided inthe dryer.

The method for producing a positive electrode active material accordingto the embodiment of the present invention makes it possible tomass-produce a positive electrode active material having stable quality.Further, since the ratio between lithium metal ratios before and afterwashing with water is high, lithium loss can be reduced, and its synergywith the above-described mass productivity makes it possible to reducethe production cost of a battery, which is essential for widespread useof lithium ion secondary batteries as power sources for electric cars.Therefore, it can be said that the industrial value of the method forproducing a positive electrode active material according to theembodiment of the present invention is very high. It is to be noted thatthe power sources for electric cars include not only power sources forelectric cars driven only by electric energy but also power sources forso-called hybrid cars also using a combustion engine such as a gasolineengine or diesel engine.

The above-described production method according to the embodiment of thepresent invention makes it possible to generate various lithium-nickelcomposite oxides, and they can be used for industrial production ofpositive electrode active materials for lithium ion secondary batteries.More specifically, a lithium-nickel composite oxide represented by thecomposition formula, Li_(x)Ni_(1−y−z)M_(y)N_(z)O₂ can be generated. Inthe above formula, M is at least one element selected from Co and Mn, Nis at least one element selected from Al, Ti, Nb, V, Mg, W, and Mo, x,y, and z preferably satisfy 0.90≤x≤1.10, 0.05≤y≤0.35, 0.005≤z≤0.05,respectively, and y and z more preferably satisfy y+z≤0.20.

When a lithium-nickel composite oxide having the above composition isproduced, a nickel compound powder used as a raw material is a powder ofa composite oxide of nickel and another transition metal (also referredto as a nickel composite oxide) or a powder of a composite hydroxide ofnickel and another transition metal (also referred to as a nickelcomposite hydroxide). These powders can be produced in accordance with aknown method. For example, a nickel composite hydroxide powder can beobtained by coprecipitating nickel, cobalt or manganese, and an additiveelement N.

Oxidization roasting of the obtained nickel composite hydroxide powderproduces a nickel composite oxide where cobalt or manganese and theadditive element N are solid-solved in nickel oxide. It is to be notedthat a nickel composite oxide can be produced also by a method in whichnickel oxide and oxides of other additive elements are ground and mixed.Application of a lithium-nickel composite oxide produced by theabove-described production method according to the embodiment of thepresent invention to a positive electrode active material for a lithiumion secondary battery can produce a high-output secondary battery havingstable quality.

A method for producing a positive electrode for a lithium ion secondarybattery using the above-described positive electrode active material fora lithium ion secondary battery including a lithium-nickel compositeoxide powder is not limited, but a positive electrode can be producedby, for example, the following method. Specifically, the powderedpositive electrode active material, a conductive material, and a bindingagent are mixed, and if necessary, activated carbon is further added.The thus obtained mixture is kneaded with a solvent used for viscosityadjustment to prepare a positive electrode mixture paste. The mixingratio among the above-described raw materials constituting the positiveelectrode mixture paste is a factor that determines the performance of alithium ion secondary battery. When the total mass of solid matter inthe positive electrode mixture except for the solvent is defined as 100mass parts, it is preferred that, as in the case of a positive electrodefor a general lithium ion secondary battery, the content of the positiveelectrode active material is 60 to 95 mass parts, the content of theconductive material is 1 to 20 mass parts, and the content of thebinding agent is 1 to 20 mass parts.

The obtained positive electrode mixture paste is applied onto, forexample, the surface of a current collector made of aluminum foil anddried to volatilize the solvent. At this time, if necessary, pressuremay be applied by a roll press or the like to increase an electrodedensity. In this way, a sheet-shaped positive electrode can be produced.The obtained sheet-shaped positive electrode is cut to have anappropriate size depending on the kind and size of a battery to beproduced, and then subjected to a battery assembly process.

Examples of the conductive material to be used include graphite (e.g.,natural graphite, artificial graphite, expanded graphite) and carbonblack-based materials such as acetylene black and ketjen black. Thebinding agent plays the role of binding active material particlestogether, and examples thereof to be used include polyvinylidenefluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine-containingrubber, ethylene propylene diene rubber, styrene butadiene,cellulose-based resins, and polyacrylic acid. In the mixture of thepositive electrode active material, the conductive material, and thebinding agent, activated carbon is optionally dispersed to increase anelectric double-layer capacity, and a solvent is added to the thusobtained positive electrode mixture to dissolve the binding agent. Asthe solvent, an organic solvent such as N-methyl-2-pyrrolidone may beused. The positive electrode mixture mixed with the solvent ispreferably kneaded by a general kneader to produce a uniform positiveelectrode mixture paste.

A negative electrode to be used as a counter electrode of theabove-described positive electrode is metallic lithium, an lithium alloyor the like, or one formed by applying, onto the surface of a metal foilcurrent collector made of copper or the like, a paste-like negativeelectrode mixture obtained by mixing a negative electrode activematerial capable of storing and releasing lithium ions with a bindingagent and further adding a solvent thereto, drying the negativeelectrode mixture, and then optionally compressing the negativeelectrode mixture to increase an electrode density. Examples of thenegative electrode active material to be used include powders of carbonsubstances such as natural graphite, artificial graphite, a fired bodyof an organic compound such as a phenol resin, and coke. As in the caseof the positive electrode, the negative electrode binding agent to beused may be a fluorine-containing resin such as PVDF, and the solventused to disperse the active material and the binding agent may be anorganic solvent such as N-methyl-2-pyrrolidone. A separator isinterposed between the positive electrode and the negative electrode.The separator is used to separate the positive electrode and thenegative electrode from each other and hold an electrolyte, and may be athin film made of polyethylene, polypropylene, or the like and having aplurality of micro pores.

A non-aqueous electrolyte solution is obtained by dissolving a lithiumsalt as a supporting salt in an organic solvent. Examples of the organicsolvent to be used include: cyclic carbonates such as ethylenecarbonate, propylene carbonate, butylene carbonate, andtrifluoropropylene carbonate; chain carbonates such as diethylcarbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropylcarbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethylmethyl sulfone and butanesultone; and phosphorus compounds such astriethyl phosphate and trioctyl phosphate, and these organic solventsmay be used singly or in combination of two or more of them. Thesupporting salt to be used may be one selected from LiPF₆, LiBF₄,LiClO₄, LiAsF₆, and LiN(CF₃SO₂)₂ or a combined salt of two or more ofthem. The non-aqueous electrolyte solution may further include at leastone selected from a radical scavenger, a surfactant, and a flameretardant.

Lithium ion secondary batteries constituted from the positive electrode,the negative electrode, the separator, and the non-aqueous electrolytesolution may be of various types such as a cylindrical type and astacked type. Irrespective of the type of lithium ion secondary battery,a lithium ion secondary battery can be assembled in the followingmanner: the positive electrode and the negative electrode are stackedwith the separator being interposed between them to form an electrodeassembly, the obtained electrode assembly is impregnated with thenon-aqueous electrolyte solution, a positive electrode current collectorand a positive electrode terminal leading to the outside are connectedusing a current collecting lead or the like and a negative electrodecurrent collector and a negative electrode terminal leading to theoutside are connected using a current collecting lead or the like, theyare accommodated in a battery case, and the battery case is hermeticallysealed.

A lithium ion secondary battery using the positive electrode activematerial according to the embodiment of the present invention has a highcapacity and a high output. Particularly, when the positive electrodeactive material according to the embodiment of the present invention isused for a positive electrode of, for example, a 2032-type coin batteryas a more preferred embodiment of the lithium ion secondary battery, the2032-type coin battery achieves a high initial discharge capacity of 165mAh/g or more and a low positive electrode resistance, and further has ahigh capacity and a high output. Further, it can be said that thermalstability is high and safety is excellent.

It is to be noted that the positive electrode resistance can be measuredby, for example, the following method. When the frequency dependence ofa battery reaction is measured by an AC impedance method generally usedas an electrochemical evaluation method, a Nyquist diagram based onsolution resistance, negative electrode resistance and negativeelectrode capacity, and positive electrode resistance and positiveelectrode capacity is obtained as shown in FIG. 1 . The battery reactionat an electrode comprises a resistance component associated with chargetransfer and a capacity component of an electric double layer. Whenthese components are represented using an electric circuit, the electriccircuit is a parallel circuit of resistance and capacity, and the entireof a battery is represented as an equivalent circuit in which solutionresistance, a parallel circuit of a negative electrode, and a parallelcircuit of a positive electrode are connected in series.

Each of the resistance components and the capacity components can beestimated by performing fitting calculation on the measured Nyquistdiagram using this equivalent circuit. The positive electrode resistanceis equal to the diameter of a semicircle on the lower frequency side inthe obtained Nyquist diagram. Therefore, the positive electroderesistance can be estimated by performing AC impedance measurement on apositive electrode produced to obtain a Nyquist diagram and performingfitting calculation on the Nyquist diagram using an equivalent circuit.Hereinbelow, the present invention will specifically be described withreference to examples, but is not limited by these examples.

EXAMPLES

A positive electrode active material was generated by the followingmethod, a secondary battery having a positive electrode produced usingthe obtained positive electrode active material was assembled, and theinitial discharge capacity and positive electrode resistance of thesecondary battery were measured to evaluate battery performance. Theevaluation of battery performance was performed by producing a 2032-typecoin batter 1 (hereinafter, referred to as a coin-type battery) having astructure shown in FIG. 2 . More specifically, the coin-type batter 1shown in FIG. 2 includes a case 2 having an almost cylindrical shape andan electrode 3 accommodated in the case 2.

The case 2 includes a hollow positive electrode can 2 a having anopening at one end and a hollow negative electrode can 2 b having anopening at one end and placed inside the positive electrode can 2 a sothat the opening is opposed to the opening of the positive electrode can2 a. By arranging the negative electrode can 2 b and the positiveelectrode can 2 a in such a manner that their openings are opposed toeach other, a space for accommodating the electrode 3 is formed by thenegative electrode can 2 b and the positive electrode can 2 a. Theelectrode 3 includes a positive electrode 3 a, a separator 4, and anegative electrode 3 b, and the positive electrode 3 a, the separator 4,and the negative electrode 3 b are stacked in this order from thepositive electrode can 2 a side so that they can be accommodated in thecase 2 in a state where the positive electrode 3 a abuts against theinner surface of the positive electrode can 2 a with a current collector5 being interposed between them, and the negative electrode 3 b abutsagainst the inner surface of the negative electrode can 2 b with acurrent collector 5 being interposed between them. It is to be notedthat a current collector 5 is interposed also between the positiveelectrode 3 a and the separator 3 c.

A gasket 2 c is provided between the periphery of the positive electrodecan 2 a and the periphery of the negative electrode can 2 b. Thepositive electrode can 2 a and the negative electrode can 2 b can befixed by the gasket 2 c so as not to come into contact with each otherto prevent relative movement between them. The gasket 2 c also has thefunction of sealing the gap between the positive electrode can 2 a andthe negative electrode can 2 b to air-tightly and liquid-tightly cut offthe inside of the case 2 from the outside.

The coin-type batter 1 for evaluation was produced by the followingmethod. First, 52.5 mg of a positive electrode active material, 15 mg ofacetylene black, and 7.5 mg of a polytetrafluoroethylene (PTFE) resinwere mixed while being ground in a mortar, and the thus obtained mixturewas press-molded at a pressure of 100 MPa to have a diameter of 11 mmand a thickness of 100 μm. In this way, the positive electrode 3 a wasproduced. The produced positive electrode 3 a was dried in a vacuumdryer at 120° C. for 12 hours before use.

The negative electrode 3 b was produced by applying a mixture of agraphite powder having a volume-average particle diameter MV of 20 μmand polyvinylidene fluoride onto a copper foil to obtain a negativeelectrode sheet and punching the negative electrode sheet to obtain adisc-shaped sheet having a diameter of 14 mm. The separator 4 was apolyethylene porous membrane having a thickness of 25 μm. As anelectrolyte solution, a mixed solution of equal parts of ethylenecarbonate (EC) and diethyl carbonate (DEC) using 1 M LiClO₄ as asupporting electrolyte (manufactured by Toyama Pure Chemical Industries,Ltd.) was used.

The coin-type batter 1 was assembled using these positive electrode 3 a,negative electrode 3 b, separator 4, and electrolyte solution in a glovebox filled with an Ar atmosphere whose dew point was controlled at −80°C. The initial discharge capacity and positive electrode resistance ofthe produced coin-type batter 1 were measured by the following methodsto evaluate battery performance.

The initial discharge capacity was evaluated in the following manner.The coin-type batter 1 was allowed to stand at room temperature for 24hours after production. After the open circuit voltage (OCV) of thecoin-type batter 1 was stabilized, a current density for the positiveelectrode was set to 0.1 mA/cm², and the coin-type batter 1 was chargedto a cutoff voltage of 4.3 V. After a 1-hour pause, the coin-type batter1 was discharged to a cutoff voltage of 3.0 V, and the capacity of thecoin-type batter 1 at this time was defined as an initial dischargecapacity.

The coin-type batter 1 was charged to a charging potential of 4.1 V andsubjected to measurement by an AC impedance method using a frequencyresponse analyzer and a potentiogalvanostat (manufactured by Solartron,1255B) to obtain a Nyquist plot as shown in FIG. 1 . The Nyquist plot isexpressed as the sum of characteristic curves showing solutionresistance, negative electrode resistance and negative electrodecapacity, and positive electrode resistance and positive electrodecapacity. Therefore, fitting calculation was performed using anequivalent circuit on the basis of the Nyquist plot to calculate thevalue of positive electrode resistance. Lithium-nickel composite oxidesaccording to examples of the present invention used as theabove-described positive electrode active material will specifically bedescribed below.

Example 1

A nickel compound powder used as a raw material was a nickel compositehydroxide powder generated by a known crystallization method. Morespecifically, a 2 mol/L mixed aqueous solution of nickel sulfate andcobalt sulfate prepared to have a molar ratio between theamount-of-substance of nickel and the amount-of-substance of cobalt of88:9 and a 1 mol/L aqueous sodium aluminate solution were continuouslydropped into a reaction tank containing a 2 mol/L aqueous sodium sulfatesolution with stirring so that the molar ratio among theamount-of-substance of nickel, the amount-of-substance of cobalt, andthe amount-of-substance of aluminum was 88:9:3. At the same time, 28%ammonia water was added to the reaction tank so that the ammoniaconcentration of the solution in the reaction tank was 5 g/L, and a 40%aqueous sodium hydroxide solution was further added so that the pH inthe reaction tank was 11.5 to 12.5. At this time, a reaction was allowedto proceed while the pH in the reaction tank was finely adjusted byadjusting the amount of the 40% aqueous sodium hydroxide solution addedso that a desired particle size of a nickel composite hydroxide could beachieved.

A slurry containing a generated nickel composite hydroxide was collectedby overflow from the reaction tank, and was filtered using a Buchnerfunnel to obtain a cake. One liter of a 40% aqueous sodium hydroxidesolution and 19 L of deionized water were added per kilogram of theobtained cake, and the resulting mixture was stirred for 30 minutes andthen filtered using a Buchner funnel to obtain a cake. Twenty liters ofdeionized water was added per kilogram of the obtained cake, and theresulting mixture was stirred for 30 minutes and then filtered using aBuchner funnel. This washing was repeated twice to obtain a nickelcomposite hydroxide cake. The obtained nickel composite hydroxide cakewas placed in a stationary hot-air dryer and dried at a dryingtemperature of 110° C. for 24 hours to obtain a nickel compositehydroxide powder. The obtained nickel composite hydroxide powder had avolume-average particle diameter MV of 11.8 μm and a carbon content of0.25 mass %.

A lithium compound powder used as the other raw material was lithiumhydroxide. The lithium hydroxide was obtained in the following manner.Lithium hydroxide monohydrate (LiOH.H₂O) was dehydrated by vacuum dryingto obtain anhydrous lithium hydroxide, and the anhydrous lithiumhydroxide was ground by a jet mill to have an average particle diameterMV of 14.1 μm. It is to be noted that the volume-average particlediameter MV of the powder was measured using a particle sizedistribution measuring device based on a laser diffraction scatteringmethod, and the carbon content of the nickel composite hydroxide powderwas measured by a high-frequency combustion-infrared absorption method.

Then, the anhydrous lithium hydroxide powder and the nickel compositehydroxide powder were weighed so that the molar ratio between theamount-of-substance of lithium and the amount-of-substance of transitionmetals other than lithium was 1.020:1.000, and were well mixed. Theobtained mixture was packed in a ceramic firing container having insidedimensions of 280 mm (L)×280 mm (W)×90 mm (H), and the firing containerwas placed in a roller hearth kiln as a continuous combustion furnace.The mixture was fired in an atmosphere gas having an oxygenconcentration of 80 vol % in a temperature pattern in which the firingtemperature measured by a thermometer provided in the furnace wasincreased from 600° C. to 765° C. at a constant temperature rise rateover about 100 minutes and then the maximum firing temperature of 765°C. was maintained for 210 minutes to synthesize a lithium-nickelcomposite oxide.

Then, 0.75 mass parts of the lithium-nickel composite oxide was added to1 mass part of pure water at 15° C. contained in a container equippedwith a stirrer and having an electric conductivity of 1 to 10 μS/cm toobtain a slurry, and the slurry was stirred for 30 minutes and thenfiltered to collect solid matter. The solid matter was placed in avacuum dryer whose chamber pressure was 0.1 kPa or less and dried at atemperature of 110° C. for 12 hours. In this way, a lithium-nickelcomposite oxide as a positive electrode active material was obtained asSample 1.

The lithium metal ratio (value obtained by dividing theamount-of-substance of lithium by the amount-of-substance of transitionmetal elements other than lithium) of the lithium-nickel composite oxideobtained as Sample 1 before washing with water was 1.030, and thelithium metal ratio of the lithium-nickel composite oxide after washingwith water was 0.993. Therefore, the ratio between the lithium metalratios before and after washing with water was 0.964 that was largerthan 0.95. From the result, it is considered that the synthesis reactionproceeded almost stoichiometrically during firing so that alithium-nickel composite oxide was generated with a little loss oflithium.

The battery characteristics of the coin-type batter 1 having a positiveelectrode produced using, as a positive electrode active material, thelithium-nickel composite oxide obtained as Sample 1 were evaluated. As aresult, the initial discharge capacity was 203.8 mAh/g. The positiveelectrode resistance expressed as a relative value determined by takingthe lowest positive electrode resistance among the positive electroderesistances of all the samples produced in Example 1 and Examples 2 and3 that will be described later as 100 was 115.

Further, lithium-nickel composite oxides were synthesized as Samples 2to 5 in the same manner as in the case of Sample 1 except that theparticle size of anhydrous lithium hydroxide after grinding was set to15.6 μm, 18.4 μm, 20.9 μm, or 25.3 μm instead of 14.1 μm, respectively,and their ratios between lithium metal ratios before and after washingwith water were determined. As a result, the ratios between lithiummetal ratios before and after washing with water of Samples 2 to 5 were0.964, 0.961, 0.958, and 0.951, respectively.

Further, coin-type batteries were produced in the same manner as in thecase of Sample 1 to evaluate battery characteristics. As a result, inthe case of Sample 2, the initial discharge capacity was 200.9 mAh/g,and the relative value of positive electrode resistance was 140, in thecase of Sample 3, the initial discharge capacity was 202.8 mAh/g, andthe relative value of positive electrode resistance was 154, in the caseof Sample 4, the initial discharge capacity was 200.3 mAh/g, and therelative value of positive electrode resistance was 205, and in the caseof Sample 5, the initial discharge capacity was 201.4 mAh/g, and therelative value of positive electrode resistance was 258.

Example 2

A nickel composite hydroxide generated in the same manner as in Example1 was roasted using a compact muffle furnace at a roasting temperatureof 400° C. for 5 hours in an air atmosphere to generate a nickelcomposite oxide.

The obtained nickel composite oxide had a volume-average particlediameter MV of 11.8 μm and a carbon content of 0.21 mass %. It is to benoted that anhydrous lithium hydroxide used was the same as that usedfor Sample 5 and having an average particle diameter MV of 25.3 μm. Alithium-nickel composite oxide was synthesized as Sample 6 in the samemanner as in Example 1, and battery characteristics were evaluated inthe same manner as in Example 1. As a result, the ratio between lithiummetal ratios before and after washing with water was 0.950, the initialdischarge capacity was 201.5 mAh/g, and the relative value of positiveelectrode resistance was 266. Further, lithium-nickel composite oxideswere synthesized as Samples 7 to 10 in the same manner as in the case ofSample 6 except that the roasting temperature during roasting using thecompact muffle furnace was set to 500° C., 650° C., 700° C., or 750° C.instead of 400° C., respectively.

As a result, in the case of Sample 7, the nickel composite oxide had avolume-average particle diameter MV of 11.5 μm and a carbon content of0.15 mass %, the ratio between lithium metal ratios before and afterwashing with water was 0.958, the initial discharge capacity was 208.6mAh/g, and the relative value of positive electrode resistance was 222.In the case of Sample 8, the nickel composite oxide had a volume-averageparticle diameter MV of 11.6 μm and a carbon content of 0.066 mass %,the ratio between lithium metal ratios before and after washing withwater was 0.963, the initial discharge capacity was 207.5 mAh/g, and therelative value of positive electrode resistance was 169. In the case ofSample 9, the nickel composite oxide had a volume-average particlediameter MV of 11.5 μm and a carbon content of 0.042 mass %, the ratiobetween lithium metal ratios before and after washing with water was0.964, the initial discharge capacity was 209.0 mAh/g, and the relativevalue of positive electrode resistance was 166. In the case of Sample10, the nickel composite oxide had a volume-average particle diameter MVof 11.5 μm and a carbon content of 0.029 mass %, the ratio betweenlithium metal ratios before and after washing with water was 0.965, theinitial discharge capacity was 207.8 mAh/g, and the relative value ofpositive electrode resistance was 154.

Example 3

A nickel composite hydroxide generated in the same manner as in Example1 was roasted using a compact muffle furnace at a roasting temperatureof 650° C. for 5 hours in an air atmosphere to generate a nickelcomposite oxide. The obtained nickel composite oxide had avolume-average particle diameter MV of 11.6 μm and a carbon content of0.066 mass %. It is to be noted that anhydrous lithium hydroxide usedwas the same as that used for Sample 3 and having an average particlediameter MV of 18.4 μm.

They were mixed under the same conditions as in Example 1, and theresulting mixture was packed in a ceramic firing container and fired ina roller hearth kiln in the same manner as in Example 1 except thatfiring was performed in an atmosphere gas having an oxygen concentrationof 80 vol % under conditions where the maximum firing temperature of550° C. was maintained for 330 minutes. A lithium-nickel composite oxidewas synthesized as Sample 11 in such a manner as described above, andbattery characteristics were evaluated in the same manner as inExample 1. As a result, the ratio between lithium metal ratios beforeand after washing with water was 0.941, the initial discharge capacitywas 197.6 mAh/g, and the relative value of positive electrode resistancewas 386.

A lithium-nickel composite oxide was synthesized by performing firingusing a roller hearth kiln in an atmosphere gas having an oxygenconcentration of 80 vol % under conditions where the firing temperaturewas increased from 600° C. to 650° C. at a constant temperature riserate over about 100 minutes and then the maximum firing temperature of650° C. was maintained for 210 minutes. Then, a lithium-nickel compositeoxide as Sample 12 was produced and battery characteristics wereevaluated in the same manner as in Example 1. As a result, the ratiobetween lithium metal ratios before and after washing with water was0.970. The initial discharge capacity was 208.7 mAh/g, and the relativevalue of positive electrode resistance was 109.

A lithium-nickel composite oxide was synthesized as Sample 13 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 80 vol % underconditions where the firing temperature was increased from 600° C. to750° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 750° C. was maintained for 210minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.971, the initial discharge capacity was209.8 mAh/g, and the relative value of positive electrode resistance was100.

A lithium-nickel composite oxide was synthesized as Sample 14 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 80 vol % underconditions where the firing temperature was increased from 600° C. to850° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 850° C. was maintained for 210minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.952, the initial discharge capacity was199.9 mAh/g, and the relative value of positive electrode resistance was254.

A lithium-nickel composite oxide was synthesized as Sample 15 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 50 vol % underconditions where the firing temperature was increased from 600° C. to765° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 765° C. was maintained for 210minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.938, the initial discharge capacity was196.9 mAh/g, and the relative value of positive electrode resistance was400.

A lithium-nickel composite oxide was synthesized as Sample 16 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 60 vol % underconditions where the firing temperature was increased from 600° C. to765° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 765° C. was maintained for 210minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.954, the initial discharge capacity was201.8 mAh/g, and the relative value of positive electrode resistance was251.

A lithium-nickel composite oxide was synthesized as Sample 17 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 70 vol % underconditions where the firing temperature was increased from 600° C. to765° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 765° C. was maintained for 210minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.957, the initial discharge capacity was202.6 mAh/g, and the relative value of positive electrode resistance was226.

A lithium-nickel composite oxide was synthesized as Sample 18 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 80 vol % underconditions where the firing temperature was increased from 600° C. to765° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 765° C. was maintained for 100minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.946, the initial discharge capacity was198.5 mAh/g, and the relative value of positive electrode resistance was311.

A lithium-nickel composite oxide was synthesized as Sample 19 andbattery characteristics were evaluated in the same manner as in the caseof Sample 12 except that firing was performed using a roller hearth kilnin an atmosphere gas having an oxygen concentration of 80 vol % underconditions where the firing temperature was increased from 600° C. to765° C. at a constant temperature rise rate over about 100 minutes andthen the maximum firing temperature of 765° C. was maintained for 150minutes. As a result, the ratio between lithium metal ratios before andafter washing with water was 0.953, the initial discharge capacity was199.7 mAh/g, and the relative value of positive electrode resistance was262.

TABLE 1 Ratio between Li Average metal particle ratios diameter CarbonFiring conditions before MV (μm) Roasting content of Max- Oxygen Limetal ratio and Battery Ni temperature Ni imum con- Maximum Before Afterafter characteristics com- of Ni compound firing centration firingwashing washing washing Initial Positive Li pound composite for temp- intemperature with with with discharge electrode Sam- hy- for hydroxidesynthesis erature atmosphere holding water water water capacityresistance ples droxide synthesis (° C.) (wt %) (° C.) (vol %) time(min) A B B/A (mAh/g) (−)   1 14.1 11.8 No roasting 0.250 765 80 2101.030 0.993 0.964 203.8 115   2 15.6 11.8 No roasting 0.250 765 80 2101.022 0.985 0.964 200.9 140   3 18.4 11.8 No roasting 0.250 765 80 2101.022 0.982 0.961 202.8 154   4 20.9 11.8 No roasting 0.250 765 80 2101.022 0.979 0.958 200.3 205   5 25.3 11.8 No roasting 0.250 765 80 2101.022 0.972 0.951 201.4 258  *6 25.3 11.8 400 0.210 765 80 210 1.0240.973 0.950 201.5 266   7 25.3 11.5 500 0.150 765 80 210 1.024 0.9810.958 208.6 222   8 25.3 11.6 650 0.066 765 80 210 1.017 0.979 0.963207.5 169   9 25.3 11.5 700 0.042 765 80 210 1.019 0.982 0.964 209.0 166 10 25.3 11.5 750 0.029 765 80 210 1.021 0.985 0.965 207.8 154 *11 18.411.6 650 0.066 550 80 330 1.022 0.962 0.941 197.6 386  12 18.4 11.6 6500.066 650 80 210 1.022 0.991 0.970 208.7 109  13 18.4 11.6 650 0.066 75080 210 1.022 0.992 0.971 209.8 100  14 18.4 11.6 650 0.066 850 80 2101.022 0.973 0.952 199.9 254 *15 18.4 11.6 650 0.066 765 50 210 1.0220.959 0.938 196.9 400  16 18.4 11.6 650 0.066 765 60 210 1.022 0.9750.954 201.8 251  17 18.4 11.6 650 0.066 765 70 210 1.022 0.978 0.957202.6 226 *18 18.4 11.6 650 0.066 765 80 100 1.022 0.967 0.946 198.5 311 19 18.4 11.6 650 0.066 765 80 150 1.022 0.974 0.953 199.7 262 (Note)Samples marked with * in the table are Comparative Examples.

The results shown in Table 1 and FIGS. 3 and 4 reveal that the ratiobetween lithium metal ratios before and after washing with watercorrelates with the initial discharge capacity and the positiveelectrode resistance, and when the ratio between lithium metal ratiosbefore and after washing with water exceeds 0.95, a positive electrodeactive material having a high discharge capacity and a low positiveelectrode resistance can stably be obtained.

REFERENCE NUMERALS LIST

1 Coin-type battery

2 Case

2 a Positive electrode can

2 b Negative electrode can

2 c Gasket

3 Electrode

3 a Positive electrode

3 b Negative electrode

4 Separator

5 Current collector

The invention claimed is:
 1. A method for producing a positive electrodeactive material for a lithium ion secondary battery which contains atleast nickel and lithium, the method comprising: a firing process inwhich a mixture of a nickel compound powder having a volume-averageparticle diameter MV of 3 μm or more and 26 μm or less and a lithiumcompound powder having a volume-average particle diameter MV of 14.1 μmor more and 26 μm or less is fired; and a water washing process in whicha lithium-nickel composite oxide powder obtained in the firing processis washed with water, wherein the firing process is performed underconditions such that a value obtained by dividing a ratio of anamount-of-substance of lithium to a total amount-of-substance oftransition metals other than lithium in the lithium-nickel compositeoxide powder after the washing with water by a ratio of anamount-of-substance of lithium to a total amount-of-substance oftransition metals other than lithium in the lithium-nickel compositeoxide powder before the washing with water exceeds 0.95.
 2. The methodfor producing a positive electrode active material for a lithium ionsecondary battery according to claim 1, wherein the nickel compoundpowder has a carbon content of 0.25 mass % or less.
 3. The method forproducing a positive electrode active material for a lithium ionsecondary battery according to claim 1, wherein the nickel compoundpowder is a powder of nickel oxide or nickel hydroxide.
 4. The methodfor producing a positive electrode active material for a lithium ionsecondary battery according to claim 1, wherein the lithium compoundpowder is a powder of lithium hydroxide or lithium hydroxide hydrate. 5.The method for producing a positive electrode active material for alithium ion secondary battery according to claim 1, wherein anatmosphere gas in which the mixture of the nickel compound powder andthe lithium compound powder is fired has an oxygen concentration of 60vol % or more.
 6. The method for producing a positive electrode activematerial for a lithium ion secondary battery according to claim 1,wherein the nickel compound powder includes a powder of a compositeoxide of nickel and another transition metal or a powder of a compositehydroxide of nickel and another transition metal.
 7. The method forproducing a positive electrode active material for a lithium ionsecondary battery according to claim 1, wherein a maximum firingtemperature in the firing process is 650° C. or higher and 850° C. orlower, and a time during which the maximum firing temperature ismaintained is 2 hours or longer.
 8. The method for producing a positiveelectrode active material for a lithium ion secondary battery accordingto claim 1, further comprising, before the firing process, apreprocessing process in which the nickel compound powder is roasted ata roasting temperature of 500° C. or higher and 800° C. or lower so asto be converted to a nickel oxide powder.
 9. The method for producing apositive electrode active material for a lithium ion secondary batteryaccording to claim 1, wherein the lithium-nickel composite oxide isrepresented by a general formula, Li_(x)Ni_(1−y−z) M_(y)N_(z)O₂ (whereinM is at least one element selected from Co and Mn, N is at least oneelement selected from Al, Ti, Nb, V, Mg, W, and Mo, x is 0.90 to 1.10, yis 0.05 to 0.35, and Z is 0.005 to 0.05).
 10. The method for producing apositive electrode active material for a lithium ion secondary batteryaccording to claim 2, wherein the nickel compound powder is a powder ofnickel oxide or nickel hydroxide.
 11. The method for producing apositive electrode active material for a lithium ion secondary batteryaccording to claim 2, wherein the lithium compound powder is a powder oflithium hydroxide or lithium hydroxide hydrate.
 12. The method forproducing a positive electrode active material for a lithium ionsecondary battery according to claim 2, wherein an atmosphere gas inwhich the mixture of the nickel compound powder and the lithium compoundpowder is fired has an oxygen concentration of 60 vol % or more.
 13. Themethod for producing a positive electrode active material for a lithiumion secondary battery according to claim 2, wherein the nickel compoundpowder includes a powder of a composite oxide of nickel and anothertransition metal or a powder of a composite hydroxide of nickel andanother transition metal.
 14. The method for producing a positiveelectrode active material for a lithium ion secondary battery accordingto claim 2, wherein a maximum firing temperature in the firing processis 650° C. or higher and 850° C. or lower, and a time during which themaximum firing temperature is maintained is 2 hours or longer.
 15. Themethod for producing a positive electrode active material for a lithiumion secondary battery according to claim 2, further comprising, beforethe firing process, a preprocessing process in which the nickel compoundpowder is roasted at a roasting temperature of 500° C. or higher and800° C. or lower so as to be converted to a nickel oxide powder.